“I oppose it now, I’ll oppose it after the election, and I’ll oppose it as president,” she said, detailing her economic plans in a speech in Michigan.

The issue has dogged her for months, including most recently at the Democratic convention last month, where a close friend of hers, Virginia Gov. Terry McAuliffe, told Politico she would find a way to get the trade deal approved once she’s in the White House. Mr. McAuliffe backed off and the Clinton campaign rejected his claims, but GOP presidential nominee Donald Trump said the comments exposed where Mrs. Clinton really stands.

She and her husband have generally been pro-trade, with former President Clinton having overseen the implementation of the North American Free Trade Agreement and the World Trade Organization, and with Mrs. Clinton in the Senate backing all but two of the trade deals she voted on in Congress.

Mrs. Clinton on Thursday said she would do more than previous administrations to enforce protections that are included in existing trade deals.

She said Mr. Trump, who has vowed to cancel bad trade deals, is unable to get anything done.

“The answer is not to rant and rave or to cut ourselves off from the world. The answer is to finally make trade work for us, not against us,” she said.